It is very easy to modify a digital image. Anyone with a basic knowledge of Photoshop software can add or remove elements of a photograph. This is not a problem if you are taking pictures for your own pleasure, but it is a serious issue for news agencies, forensic photographers and insurance companies. Most newspapers, for example, don’t allow anything other than basic retouching (dust removal, for example) and photographers have been fired for breaking the rules. But how can you tell if an image has been changed? Canon has one solution – data verification.

One advantage of the Canon system is that it costs the photographer nothing. There are no accessories to buy and no software to install. Everything you need is already available in the camera – providing you have one of the compatible models. All you have to do is switch on the data verification system. It is the end user of the image – the news agency or police department – which needs to buy the Canon Data Verification Kit to check the integrity of your image.

Code generator

When you take a digital photo with certain EOS cameras, a code can be generated and attached to the image file. This code is based on the digital content of the image. At any time in the future, a Canon Data Verification Kit can be used to regenerate a code from the same image, or a copy of that image. If the first and second codes match, the image has not been modified. If the codes are different, it means that the content of the image has been changed.

The system is so precise that even a discrepancy of one bit between the two images will throw up an error. A bit is the smallest element of an image (there are 8 bits to a byte, and 1,048,576 bytes to a megabyte), so the system will pick up any change, however small. Here is how it works.

- Data from the image captured by the camera is compressed. This data is then arithmetically processed using a MAC (Message Authentication Code) secret key to create ‘Signature data A’. This is attached to the image data file as additional digital information.

- The verification system separates ‘Signature data A’ from the image file. The file is then processed again using the same MAC secret key that was assigned when the photo was taken. This produces a new file that is labelled ‘Signature data B’.

- If ‘Signature data A’ and ‘Signature data B’ match, then the image data has not been altered. Any difference between the two files shows that the second file is a modified version of the first. Since the same method can be used to verify whether image header information has been tampered with, the reliability of both the image and the attached information can be verified at the same time.

Compatible cameras

The great thing about data verification is that – as a photographer – it does not cost you a penny, providing you have a compatible camera. All you have to do is switch on the code generating system built into the camera and keep shooting. It is the newspaper picture desk or law enforcement agency that needs the data verification kits if they want to check the integrity of images.

The following cameras are compatible with the Canon Data Verification kits. The information in brackets shows the Personal Function or Custom Function number needed to activate the data verification system:

EOS-1Ds (P.Fn-31)
EOS-1Ds Mark II (P.Fn-31)
EOS-1Ds Mark III (C.Fn 1V-15)
EOS-1D Mark II (P.Fn-31)
EOS-1D Mark II N (P.Fn-31)
EOS-1D Mark III (C.Fn IV-15)
EOS 20D and 20Da (C.Fn-18)
EOS 30D (C.Fn-19)
EOS 5D (C.Fn-20)

Personal Functions (P.Fn) can only be set from a computer. Install File Viewer Utility (or EOS Viewer Utility) from the Digital EOS Solution Disk supplied with the camera. Connect the camera to the computer using the supplied interface cable and launch the viewer utility application. Click the Camera Settings button, then the Detailed Settings tab, then the Personal Functions button. In the new window, select the last of the Other Functions tabs at the right of the screen. Under P.Fn-31 tick the box for ‘Add original decision data’.

Custom Functions (C.Fn) can be set via the Menu screen on the back of the camera. Select Custom Functions from the Tool menu, then the appropriate custom function number (see above). Change ‘Add original decision data’ from ‘Off’ to ‘On’.